Thermal switch



April 1, 1941. g R ER 2,236,6Q7

THERMAL SWITCH Filed Sept. 3, 1938 3 Sheets-Sheet l Inventor: Leo R.Deters,

His CAttorrwey.

April 1, 1941, L R PETERS 2,236,97

THERMAL SWITCH Filed Sept. 3, 1938 3 Sheets-Sheet 2 A rifl 1, 19 M L. R.PETERS zpzggyfim THERMAL SWITCH Filed Sept. 5, 1933 3 Sheets-Sheet 3Inventor: Leo F2 et er s,

Patented Apr. 1, 1941 THERMAL SWITCH Leo R. Peters,

East Cleveland, Ohio, assignor to General Electric Company, acorporation of New York Application September 3, 1938, Serial No.228,365

11 Claims.

My invention relates generally to switches for making and breakingelectrical circuits, and particularly to switches of the thermostatictype in which a bimetallic strip is adapted to be flexed by the heatradiated by a heater element connected in the circuit to thereby breakthe circuit through the bimetallic strip. Still more particularly, myinvention relates to a switch of the above type for operating electricdischarge lamps and the like.

In the operation of gas-filled electric discharge lamps havingthermionic electrodes, it is necessary to provide a suitable startingcircuit for the purpose of initiating the arc dischargebetween the lampelectrodes. Before the arc discharge can take place, the electronemissive substance with which the lamp electrodes are coated must bepreheated to a temperature at which it will become suflicientlyactivated to readily support the arc discharge.

In one type of circuit employed for starting and operating gas-filledelectric discharge lamps having thermionic electrodes, one currentsupply wire of each electrode is connected to one of the terminals of asource of alternating current in series with a choke coil and a mainswitch. The other current supply wires of the electrodes areinterconnected by an auxiliary switch. When the discharge lamp is putinto operation, both the main and auxiliary switches are closed and thethermionic electrodes are connected in series through the choke coil tothe source of current so that they are traversed by a current by whichthey are preheated. When these electrodes are preheated to a temperaturesufiicient to activate the electron-emissive material thereon, theauxiliary switch is opened so as to interrupt the series connection of'the'electrodes and initiate the arc discharge therebetween. Theinitiation of the arc discharge is facilitated by the voltage impulseproduced in the choke coil due to the opening of the auxiliary switch.During the continuance of the arc discharge, no separate heating currentis passed through the electrodes inasmuch as the heating thereof isthereafter efiected by the arc discharge current alone.

Where the main switch for operating an electric discharge lamp is to bemounted remote from the lamp itself, it is highly desirable, in order tosimplify the wiring arrangement, that the auxiliary switch referred toabove for starting lamp operation be mounted either in the lamp fixtureitself or in close proximity thereto. In addition, it is also desirablethat the said auxiliary switch be automatic in operation so that the arediszatlon of the heat radiated by the said heater elecharge in the lampwill be automatically initiated upon closure of the main or line switch.Accordingly, one object of my invention is to provide a thermostaticauxiliary switch, simple in construction and positive in operation, forautomatically starting and operating electric discharge lamps havingthermionic electrodes therein upon closure of the main switch.

Another object of my invention is to provide an efficient bimetallicstarting and operating switch for electric discharge lamps which can bemanufactured inexpensively and which is extremely compact and of minimumdimensions.

Still another object of my invention is to provide a thermostatic switchof the bimetallic type in which the heat-actuated bimetallic element isdisposed around the heater element so as to surround the same to therebygain maximum utiliment and to readily obtain the desired movement of thebimetallic element.

A further object of my invention is to provide a bimetallic switch witheffective temperature compensating means to maintain the switch contactsin their normal positions relative to one another for a protractedperiod of time following the energization of the switch heater element.

A still further object of my invention is to provide, in a bimetallicswitch for starting and operating electric discharge lamps havingthermionic electrodes, a positive delayed action feature for prolongingthe period of initial heating of the lamp electrodes so that they becomesufficiently electron-emissive to support an arc discharge therebetween.

Still another object of my invention is to provide a bimetallic switchhaving a bimetallic element formed with a portion substantiallyunaffected initially by the switch heater element whereby a time lag isproduced in the movement of such portion, the subsequent movement ofsuch portion thereafter combining with the initial deflection of thebimetallic element to thereby maintain the switch contact members intheir operative or heat-actuated relation with respect to one anotherdespite an appreciable drop in the current flow through the switchheater element.

A further object of my invention is to provide abimetallic switch whichwill cool rapidly upon de-energization of the switch heater element,thus permitting a recommencement of circuit operations in a relativelyshort period of time following such de-energization of the switch heaterelement.

Further objects and advantages of m: invention will appear from thefollowing detailed de heat-actuated contact member of the switch.

scription of species thereof and from the accompanying drawings inwhich:

Fig. 1 is a front view, partly in section, of a bimetallic switchcomprising my invention; Fig. 2 is a. side view, partly in section, ofthe switch shown in Fig. 1; Fig. 3 is a fragmentary isometric view, onan enlarged scale, of the switch shown in Figs. 1 and 2; Fig. 4 is adiagrammatic illustration of a wiring arrangement utilizing a bimetallicswitch according to my invention for starting and operating an electricdischarge lamp; Fig. 5 is a fragmentary side view, on an enlarged scale,of the bimetallic switch shown in Figs. 1 to 3 with the successiveoperative positions of the bimetallic switch elements shown in dottedvlines; Figs. 6 and 7 are fragmentary front and side views respectively,on an enlarged scale, of a modified form of bimetallic switch accordingto the invention; Figs. 8, 9 and 10 are front. side and rear views,similar to Figs. 6 and 7, of another modification of my invention; Fig.11 is a diagrammatic illustration of a wiring arrangement utilizing abimetallic switch according to the form shown in Figs. 8 to 10 forstarting and operating an electric discharge lamp; Figs. 12 and 13 arefront and side views, similar to Figs. 6 and '7. of still anothermodification of my invention; Fig. 14 is a front View, similar to Fig.12, of another modification of the invention; and Figs. 1 5 and 16 arefront and side views respectively, similar to Figs. 6 and '7, of afurther modification of my invention.

Referring to Figs. 1 to 3, the bimetallic switch there shown comprises asealed elongated container I 0, preferably of glass, provided at one endwith an inwardly extending stem II having a press portion I2. Aplurality of current supply or leading-in wires, comprising a front pairI3, I4 and a rear pair IE, IS, extend into the container I and haveportions thereof sealed in the press portions I2 of the stem orsupporting member II. A heater element I'I, comprising a closely coiledwire of suitable refractory material, preferably Nichrome wire, issupported by, and connected at its opposite ends to, the lead wires I3and I6. As is evident from the drawings, theheater coil H is so mountedas to extend transversely of the envelope or container I0.

A pair of switch contact members I8, I9 are secured at one end.preferably by welding, to the leading-in wires I4 and I respectively.

Each of the said contact members I8, I 9 consists of a length ofbimetallic strip, such as the commercially obtainable bimetal of chromeiron and Invar, approximately 0.008 of an inch thick, 0.05 of an inchwide, and three-quarters of an inch long. As shown, the bimetallicelements are mounted so as to extend substantially longitudinally of thecontainer III. The bimetallic element I8 is bent intermediate its endsinto a hook or half-loop portion 20, the open end of which preferablyfaces in a direction parallel to the length of the said element andtowards the free or unsupported end thereof. This hooked portion 20accordingly divides the bimetallldelement I8 into two separate legportions, to wit: a lower or main leg portion 2I and an upper or freeleg. portion 22. The curved portion of the hook 20 is of slightly largerdiameter than that of the heater coil I1. and is so located with respectto the said heater coil as to be concentric therewith, thus partiallysurounding the same. The bimetallic element I8 accordingly forms the Theother bimetallic element I9, hereinafter referred to as the springcontact member, is disposed so that the free end thereof engages thefree end of the heat-actuable element I8 when the latter is in itsnormal or unheated position. The actual contact, however, between thetwo bimetallic elements is through a pair of contacts 23, 24 of a silveralloy, preferably coin silver, provided on the-free ends of the membersI8 and I9 respectively. As clearly shown in Fig. 5, the spring contactmember I9 is so mounted on the leading-in wire I5 as to be flexed fromits normal position A (indicated by the dotted lines) by theheat-actuable element I8 when the latter is in its normal or unheatedposition. The member I 9 thus forms a temperature compensating elementor device serving to maintain the ends of the two contact members I8, ISin positive engagement during ambient temperature changes. The amount offiexure of the member I9 from its normal position by the heat-actuableelement I8 is in this case so proportioned that the contact between thetwo members I8, I9 is broken in two to three seconds when a current ofone-quarter of an ampere is passed through the heater element I1 and thesaid members are surrounded by an atmosphere of air.

To obtain rapid cooling of the switch parts upon deenergization of theswitch heater element H, the container I0 is filled with a suitable gasof high heat conductivity, preferably hydrogen or helium, at a pressureof approximately 4 mm. of mercury or thereabouts. I have found that apressure of 4 mm. of the gaseous filling provides'a maximum coolingeffect while still affording adequate protection against arc-over. Therapid cooling provided by such a gaseous filling consequently permitsoperation of the switch in a very short period of time, approximatingthree or four seconds, following the deenergization of the heater coilii.

Referring now Ito Fig. 4, illustrating diagrammatically the manner inwhich a bimetallic switch according to my invention may b used to startand operate an electric discharge lamp 25 having thermionic electrodm 2Band 21, one end of a choke coil 28 is connectedby lead 29 to one end ofthe heater element I! of the bimetallic swritch I0. The other end of thechoke coil is connected by lead 30, a main switch 3|, and lead 32 to oneside of the line. Th opposite end of the heater element I1 is connectedby lead 33 to one end of the lamp electrode 28, while one end of theopposite electrode 2'! is connected by lead 34 to the other side of theline. The ends of the electrodes 26, 21 opposite to those ends connectedto the heater coil I1 and current source are connected to the bimetalliccontact members I 8, I 9 of the bimetallic switch III, electrode 26being connegated by lead 35 to the bimetallic element I8 and electrode21 being connected y lead 36 to the bimetallic element I9. The etal licelements I8, I8 thus function as a switch for connecting the electrodes26, 21 in series across the line. A small condenser 31, of approximately0.05 mfd. capacity. is preferably connected across the switch contactmembers I-8, I9 so as to shunt the same.

As before stated, the heat-actua'ble bimetallic element I8, in itsnormal or inoperative position, is in electrical contact with the otheror spring bimetallic element I9, thereby interconnecting the lampelectrodes in series across the line. To start lamp operation, the mainswitch 3| is closed.

aaaaeer bimetallic element towards the position B indi-' baited bydotted lines in Fig. 5. The contact, however, between the two bimetallicelements II, is is not broken during the initial stages this movement orthe element l8 since the element is, due to the fiexure thereof from itsnormal position A by the element I8, tends to assume the said normalposition and s0 follows the movement of the leg 22. The continuedmovement, however, of the upper leg portion 23 towards the position Eeventually results in the separation oi the two bimetallic elements i8,i9 and the braking of the contact therebetween. As previously stated,the two bimetallic elements are so arranged with respect to one anotherthat the period of time required for the two elemenm to actuallyseparate is approximately three seconds when the current passing throughthe switch heater coil i'i amounts to a quarter of an ampere. Thisinterval of time, during which the seriesconnected lamp electrodes 26,21 are traversed by a current by which they are heated, is sufficientlylong to permit the said electrodes to be preheated to arc-sustainingtemperature. The interruption of the series connection 0! the electrodes2S and 21 by the separation of elements l8, l-9 immediately initiatesthe arc discharge in the lamp. The initiation of this arc discharge isfacilitated by the voltage impulse produced in th choke coil 28 byreason of the abrupt interruption of the electrode series connection.

When the arc discharge in the lamp commences, the current flow throughthe lamp circuit automatically drops to a lower value. Thus, in presentcommercial electric discharge lamps of the fluorescent type, the currentflow drops from approximately 0.5 of an ampere before initiation of thearc discharge to approximately 0.27 of an ampere after the arc dischargeoccurs, and in some cases to as low as 0.20 of an ampere. Consequently,the intensity of the heating effect of the heater coil ll decreases aproportional amount. As a. result, the upper leg portion 22 of I i8according to the invention. Thus, by the time the upper leg portion 22actually separates from bimetallic element l9 during the initialmovement of such leg portion, the lower or main leg portion 21 hasbecome sufliciently heated, essentially 'by conduction of heat from thehook portion 20, to cause a. movement of the said main leg portion 2! tothe position C indicated 'by dotted lines in Fig. 5. This subsequentmovement of the main leg portion 2! accordingly adds to the deflectionof the upper leg portion 22 and tends to neutralize the return movementof such upper leg portion towards the element l9, which return movementoccurs when th arc discharge in the lamp commences. The additionalmovement thus provided by the main leg portion 2i consequently insuresthe maintenance of the separated relation of the two bimetallic elementsII, I! and thereiore the continuance of the are discharge in the lamp.The particularly shaped construction of the element l8 accordinglyprovides a time-delayed compensating arrangement whereby the spacedrelation of the switch contact points 23, 2 is maintained despite an81D- preciable drop in the current flow through the switch heater coilupon the initiation of the arc discharge in the lamp.

When the main switch 3| is opened to discontinue further operation ofthe lamp, the current passing through the heater coil l'l ceases so thatthe bimetallic element i8 gradually cools and returns to its normalposition. If, in the case of a conventional bimetallic switch, the mainswitch 3| is closed to restart the lamp before the element i8 hasreturned to the position where it re-= engages element l9, then the veryinstant ele= ment l8 engages element is so as to reestablish the circuitthrough the heater coil ii and the lamp electrodes 26, 2?, the slightreheating oil element 58 by the heater coil ll immediately separates thetwo bimetallic elements id, id without allowing sufllcient time for theelectrodes 2d, 27; to become adequately preheated to are discharge=supporting temperature. As a result, the lamp fails to start. This samecondition occurs if the main switch 3! is closed exactly at the momentthe element l8 has just re-engaged element I5. If the main switch isleft closed, the above described action is continually repeated againand again, the element i8 alternately making and immediately breakingthe circuit through the electrodes 26, 21 before they have becomesufficiently preheated to support the arc discharge in the lamp. Toactually restart the lamp, it is necessary to keep the main switch openuntil the element !8 has returned to a position sufllcientlyintermediate the position at which it engages element l9 and its normalor unheated position. The con-tact between the two elements I8, I 9 isthen maintained, after closure of the main switch SI, for a. sufllcientlength of time to permit adequate preheating of the lamp electrodes.

With a switch constructed according to my invention, and with thestarting arrangement shown in Fig. 4, the above-mentioned condition ismaterially rectified, so that the lamp will start even though the mainswitch 3| be closed immediately following the opening of the same. Theelimination of such an undesirable condition is accomplished by the useof coin silver for the contact points 23, 240i the elements I8 and i9,together with the condenser 31 connected across the said elements. If,after discontinuation of lamp operation and before the element 18 hasre-engaged element IS, the main switch 3! is closed, then the condenser31 induces a spark of relatively high intensity between the approachingcontact points which tends to weld the said contacts and causes them tostick together. Since the interengagement of th two contact points 23,24 re-establishes the circuit through the heater coil H and electrodes28, 27!, the consequent reheating of the bimetallic element l8 tends toseparate the same from the other element l9. However, the tendency ofthe contact points to stick together resists such separation until theforce tending to separate element l8 from element 09 is sufilcient toovercome the said sticking tendency. Thus, after closure of the circuitthrough th lamp electrodes 26, 21. a time lag occurs before the saidcircuit is interrupted. This time lag is of sufficient duration topermit adequate preheating of the lamp electrodes 26, 21 to aredischarge-sustaining condition. Accordingly, when the contact points 23,24 actually do separate so as to break the series-connection of the lampelectrodes, the arc discharge in the lamp will therefore commence. It isthus evident that the use of coin silver contact points together withthe condenser 31 permits rapid and positive restart of lamp operation.

Referring now to the modification shown in Figs. 6 and 7, the bimetallicswitch there shown is the same as that shown in Figs. 1 to 3 except forthe particular construction of the heat-actuated bimetallic element It.Thus, instead of the hooked portion 28 surrounding the heater elementll, the element l8 in the modification is formed with a double coilsurrounding the said heater element, the double coil consisting of aninner coil 39 and an outer coil 40. As shown, the outer coil 40 isformed as a continuation of the main leg portion 2| of the element l8,while the inner coil 39 is formed by reversely bending the element l8back upon itself from the end of the outer coil 40, the inner coil 39thu forming a continuation of the upper or free leg portion 22 of theelement It. The two coils 39 and 4B are t so arranged as to besubstantially concentric with one another, and are preferably spaced aslight amount.

The method of connecting this modified form of bi-metallic switch in thelamp circuit, and the operating characteristics of the'said switch, arethe same as that described in connection with the first form of switch.When the main switch 31 is closed, the heating of the inner coil .39 ofthe bimetallic element It by the heater element it causes the upper legportion 22 to bend towards the position B (Fig. 5), the other bimetallicelement I 9 moving along with element l8 during the first stages of itsmovement to thereby maintain the contact therebetween. The continuedmovement, however, of the upper leg portion 22 eventually causes aseparation of elements l8 and I9 and the initiation of the arc dischargein the lamp. By the time the leg portion 22 reaches the position B, theouter coil til and also the main leg portion 2!. have begun to heat up,by conduction of heat from the inner coil 39 and otherwise.Consequently, the resulting movement of the heated outer coil 60, andthe main leg portion 2| to position C (Fig. 5) tends to neutralize anyreturn movement of the upper leg portion 22 towards element It, thusmaintaining the separated relation of elements l8, l9 and so insuringthe continuance of the arc discharge in the lamp.

The modification illustrated in Figs. 8 to 10 difiers from the form ofswitch shown in Figs. 1 to 3 in the construction of the bimetallicelement l9 (corresponding to element it in Figs. 1-3) and in the use ofa second heater element to actuate the said element l9. Referring toFigs. 8 to 10, a heater coil il, similar to the heater coil II but ofrelatively lower wattage, is mounted in a like manner between theleading-in wire it and a support wire 62 extending from, and embeddedin, the press portion l2 of the stem H. The bimetallic element I9 issecured to this auxiliary support wire 62 and is provided with a hook orU-shaped portion 43 similar to the hook portion of element H8. Thebimetal in element I9 is reversed, however, from that in element It, sothat the hooked portion 43, on heating, tends to open or uncoil insteadof closing, as in the case of the hooked portion 20. In this manner, theupper leg portions 22, 22 of the two bimetallic elements It, I9 move inthe same direction and in substantial unison when the said elements areheated by their respective heater coils II and ti, thus tending tomaintain the interengagement of the two elements. However, because 01'the difference in wattage of the two heater, coils i1 and ll, the latterbeing of lower wattage than the former, a difierential movement of theupper leg portions 22, 22 of the bimetallic elements occurs. In otherwords, the leg portion 22 of the element I9 does not move as fast as theleg portion 22 of element I8, so that there is a tendency for the saidelements to separate. This separation, however, is delayed by somounting the elements l8, IS with respect to one another that theelement i9 is under a slight spring tension similar to that described inconnection with Figs. 1 to 3. The separation of the contact points 23,24 therefore does not take place the instant current passes through theheater coils it and H, but instead is delayed until the differential ofmovement between the two bimetallic elements l8, l9 exceeds the amountof spring tension, when separation actually does occur.

Referring to Fig. 11, the circuit employed for starting and operatingelectric discharge lamps with a bimetallic switch according to themodification of Figs. 8 to 10 is very similar to the circuit shown inFig. 4, the only difference being the interposition of the heater coiliii in the circuit connecting the lamp electrodes in series. Thus, asshown in Fig. 11, one end of the heater coil BI is connected by lead 64to the bimetallic element l9, while the other end is connected by leadfifi to one end of electrode 21. The operation of this modified form ofbimetallic switch in starting' and in restarting lamp operation is thenthe same as that described in connection with the first form of switch.

The modified form of bimetallic switch shown in Figs. 12 and 13 issimilar to that shown in Figs. 1 to 3, the only difference being in theconstruction of the bimetallic element it. In Figs. 12 and 13, theelement i8 is formed with a helical coil 68 surrounding the heaterelement ll, instead of the hooked portion 20 present in Figs. 1 to 3.

I This helical coil construction of element 18 does not provide thetime-delayed compensating feature characteristic of the previous formsof the invention. Accordingly, care must be taken in adjusting the twobimetallic elements 58 and it with respect to one another so that theseparation of the said elements is maintained once the arc discharge inthe lamp has been initiated, at which time the current flow through theheater element ll drops to nearly half its former value and in somecases even more.

In Fig. 14, the modification there shown difiers from the switch shownin Figs. 12 and 13 only in that the heater element I! is enclosed withina mass of refractory material, preferably in the form of a cylinder 17of porcelain or glass, which is in turn surrounded by the helical coil45 in the bimetallic element It. The heater element I'i heats thisrefractory mass 41 which in turn heats the surrounding helical coilportion 66 of the bimetallic element l8. The time required for therefractory mass 61 to heat up accordingly produces a time lag in theheating of the bimetallic element 18. Consequently, the movement ofelement I8, and resulting separation of the same from element i9, isdelayed for a definite interval of time following the closure of thecircuit through the heater element ll, thus permitting the lampelectrodes to heat up to arc-supporting temperature.

The construction shown in Figs. 15 and 16 is a slight modification ofthe switch illustrated in Figs. 12 and 13, the only difierence being inthe construction of the spring contact element H9 (corresponding toelement IS in Figs. 12 and 13) which in this modification is formed witha helical coil 48 the same as the helical coil 46 in element l8. The twobimetallic elements l8, 9 are therefore exactly alike in construction,thus greatly simplifying manufacture of the switch. A further butinconsequential difierence of this form of switch from the previousforms is in the connection of one end of the switch heater element I! toleading-in wire ll instead of IS, and the connection of the bimetallicelement II to leading-in wire l6 instead of i4. I

It will be noted that in all the above-described forms of bimetallicswitches, the bimetal surrounds the heater coil l1 instead of the lattersurrounding the bimetal, as in the case of conventional switches of thistype. The disposition of the bimetal around the heater coil provides adefinite control over the movement of the bimetal with respect to timeand with respect to varying current flow through the heater coil. Thusby' suitably shaping the bimetal around the heater coil it is possibleto obtain the desired character of movement of the bimetal with respectto time and with respect to varying current fiow. The forming of thebimetal around the heater coil also greatly simplifies the manufactureof the switch, since it permits the use of a machinewound heater coil.Furthermore, such construction permits the use of a higher ratio betweenthe lamp starting and operating wattage input, allowing sufiicient timefor initiation of the arc discharge in starting the lamp and stillmaintaining the switch contact members separated, under the decreasedcurrent fiow existing during lamp operation, so as not to interrupt thecontinuance of the arc discharge.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A bimetallic switch comprising a support member, a heater elementmounted on said support member, and a pair of co-operating contactmembers secured at their base ends to said support member, one of saidcontact members comprising a bimetallic strip so formed as to provide anintermediate portion at least partly surrounding the said heater elementin relatively close relation thereto and having its base portionsubstantially unaffected initially by the said heater element butsubsequently deflected, by the heat generated by said heater element, ina direction adding to the deflection of the free end portion of saidbimetallic strip.

2. A bimetallic switch comprising a support member, a heater elementmounted on said support member, and a pair or co-operating bimetalliccontact members secured at one end to said support member and normallyin engagement with each other at their unsupported ends, one of saidbimetallic contact members having an intermediate portion so formed asto at least partly surround the said heater element, and the other ofsaid bimetallic contact members being flexed a slight amount from itsnormal position by the said one of said bimetallic contact members.

3. A bimetallic switch comprising a support member, a pair of heaterelements mounted on said support member. one of said heater elementsbeing of relatively high wattage capacity and the other being ofrelatively lowwattage capacity, and a pair of co-operating bimetalliccontact members secured at one end to said support member, one of saidbimetallic members having an intermediate portion so formed as to atleast partly surround the said high wattage heater element and the otherof said bimetallic members likewise having an intermediate portion soformed as to at least partly surround the said low wattage heaterelement, said bimetallic members being so arranged with respect to oneanother as to move in the same direction under the influence of liketemperature changes therein. 4. A bimetallic switch comprising acontainer, a heater element mounted within said container, and a pair ofco-operating contact members mounted within said container, at least oneof said contact members comprising a bimetallic strip having anintermediate portion disposed closely adjacent the said heater element,said container having a filling of a gas of high heat conductivity at apressure of approximately four millimeters.

5. A bimetallic switch comprising a support member, a heater elementmounted on said support member, and a pair of co-operating contactmembers mounted on said support, one of said contact members comprisinga bimetallic strip having at least a portion thereof disposed closelyadjacent said heater element, said contact members havinginterengageable contact points of a silver alloy which tend to stickwhen brought together in a heated condition, and a condenser ofrelatively low capacity connected across the said contact members andcapable, when said contacts are separated and a'potential is appliedacross the condenser, of inducing a spark of sufficient intensitybetween the contacts to cause them to stick together.

6. A bimetallic switch comprising a support member, a heater elementmounted on said support member, and a pair of co-operating contactmembers, one of said contact members comprising a bimetallic striphaving an intermediate portion formed to at least partially surroundsaid heater element in relatively close relation thereto and also havinga fixed end portion and a free end portion extending in oppositedirections a substantial distance away from said intermediate portion,the end of said fixed end portion being secured to said support, theother contact member being arranged to engage with the free end portionof said bimetallic strip.

7. A bimetallic switch comprising a support member, a heater elementmounted on said support member, and a pair of co-operating contactmembers, one of said contact members comprising a bimetallic striphaving an intermediate portion formed to at least partially surroundsaid heater element in relatively close relation thereto and also havinga fixed end portion and a free end portion extending in oppositedirections a substantial distance away from said intermediate portion,the end of said fixed end portion being secured to said support, theother contact member also comprising a bimetallic strip secured at oneend to said support and extending in the same general direction as saidfirst-mentioned bimetallic strip with its free end arranged to engagethe free end of said firstmentioned bimetallic strip.

8. A bimetallic switch comprising a support member, a heater elementmounted on said support member, and a pair of co-operating contactmembers each secured at one end to said support member, one of saidcontact members comprising a bimetallic strip having a base portionextending from said support member to a position adjacent said heaterand an intermediate portion bent back toward said support in the form ofa U partially surrounding said heater element and v a free end portionextending a substantial distance beyond said heater element, the othercontact member also comprising a bimetallic strip extending from saidsupport member in the same general direction as said first-mentionedbimetallic strip with its free end arranged to engage the free end ofsaid first-mentioned bimetallic strip. i

9. A bimetallic switch comprising a support member, a heater elementmounted on said support member, and a pair of co-operatingbimetallic-contact members secured at their base ends to said supportmember and normally in engagement with each other at their unsupported.ends, one of said bimetallic members 'being so formed as to provide anintermediate contact members being flexed a slight amount from itsnormal position by the said one of said bimetallic members.

10. A bimetallic switch comprising a container, a heater element mountedwithin said container. and a pair of co-operating contact membersmounted within said container, at least one of said contact memberscomprising a bimetallic strip having an intermediate portion disposedclosely adjacent the said heater element. said container having afilling at a pressure oi approximately 4 mm. of a gas included withinthe group consisting of hydrogen and helium.

11. A bimetallic switch comprising a support member, a heater elementmounted on said support member, and a pair of co-operating contactmembers mounted on said support, one of said contact members comprisinga bimetallic strip having at least a portion thereof disposed closelyadjacent said heater element, said contact members havinginterengageable contact points of a coin silver alloy, and a condenserof relatively low capacity connected across the said contact members andcapable, when said contacts are separated and a potential is appliedacross the condenser, of inducing a spark of sufllcient intensitybetween the contacts to cause them to stick together.

LEO R. PETERS.

